This invention relates to an artificial leg having a knee mechanism comprising a multi-bar linkage.
The use of multi-bar linkages, particularly four-bar linkages, in the knee mechanisms of artificial legs is well-known. Six-bar linkages have also been used but are less common.
Normally, a knee mechanism incorporating a multi-bar linkage has an axis of rotation whose position moves according to the degree of flexion of the knee. This feature has been employed by designers to produce a leg in which the axis of rotation is relatively high at full extension and descends towards the natural knee centre with increasing flexion of the knee. A leg having these characteristics is of particular use for a patient with a weak thigh stump, since a high initial instantaneous axis of rotation enables the patient to initiate flexion with relatively little muscular effort. If the initial axis is sufficiently high, the leg can be made to have positive stability at heel contact, i.e. at full extension.
However, a linkage with a moving axis of rotation as described above has the disadvantage that, during the initial stage of flexion from the fully extended position, the lower end of the thigh moves forward appreciably in a near-linear manner relative to the top of the shin. This can cause considerable difficulty in obtaining a neat cosmetic appearance in the region of the knee.
It is relatively easy to provide a reasonable cosmetic appearance with a leg having a uniaxial knee mechanism, i.e. a mechanism with a single, fixed axis of rotation. Therefore, in cases where the patient is relatively strong and can exert sufficient muscular control, the more common uniaxial mechanism with a single rotating bearing may be preferred. However, a patient with a disarticulated knee amputation ("through-knee amputation") cannot use the normal internal uniaxial mechanism since the stump extends into the space needed to accommodate the mechanism. The same applies to a patient with an above-knee amputation having a very long stump; there is insufficient room for even the most compact of known internal uniaxial knee mechanisms. In such cases it is conventional to provide a leg with external side joints. In comparison with a leg having an internal joint, one with side joints is unsightly, and has the further disadvantage that it is difficult to incorporate effective or neat control mechanisms, for example a knee lock or a swing control.